Integration process principles for maximizing the boil off recovery on a h2 liquefier plant

ABSTRACT

A method for recovering boil-off gas from a system including one or more liquefaction trains including transport trucks or loading bays, a gaseous hydrogen feed stream, a lower-temperature cold box, and a low-pressure hydrogen compressor. The method including collecting a boil-off gas stream from the transport trucks or loading bays, determining the pressure of the boil-off gas stream, and depending on the pressure, recycling the boo-off gas stream to predetermined destinations. Wherein the boil-off gas stream has either a low-pressure, having a pressure of less than 2 bara, or a medium-pressure, having a pressure equal to or greater than 2 bara.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119(a) and (b) to U.S. Provisional Patent Application No. 63/248,185, filedSep. 24, 2021, the entire contents of which are incorporated herein byreference.

BACKGROUND

Hydrogen liquefaction is a process well known in the art. Onenon-limiting example is shown in FIG. 1 . Hydrogen stream 105 isintroduced into higher-temperature cryogenic cold box 106. In thiscontext, “higher-temperature” is defined as a cold box that operates ator near a temperature of 80 Kelvins (−193 C). With “at or near” beingdefined as meaning plus or minus 20 C, preferably plus or minus 15 C,and more preferably plus or minus 10 C. Higher-temperature cold box 106is refrigerated by a nitrogen refrigeration cycle illustratedsimplistically by nitrogen compressor 107, and thus produces coldhydrogen stream 108. Cold hydrogen stream 108 is then introduced intolower-temperature cryogenic cold box 109. In this context,“lower-temperature” is defined as a cold box that operates at or near atemperature of 20 Kelvins (−253 C). Lower-temperature cold box 109 isrefrigerated by a hydrogen refrigeration cycle illustratedsimplistically by low-pressure hydrogen compressor 110 and high-pressurehydrogen compressor 111, and thus produces liquid hydrogen stream 112.The skilled artisan will recognize that there are numerous other systemsfor liquefying hydrogen and will also recognize that FIG. 1 is a verysimple representation of a much more complex system. For example, manysystems utilize one or more refrigeration cycles that utilize mixedrefrigerant for precooling purpose or Helium refrigerant for Hydrogenliquefaction.

Hydrogen liquefaction is an expensive process and the processing ofboil-off gas (BOG) for the recovery of vaporized hydrogen molecules isextremely important. This is especially true when the gaseous hydrogenis produced at a high cost, for example through electrolyzes. BOGgeneration generally occurs through the liquefier cold end flash, liquidhydrogen piping, liquid hydrogen storage, during the truck loadingprocess and afterwards during the delivery of the liquid hydrogen.

There are different ways known in the art to recover the BOG from thosedifferent areas of the plant (liquefier, storage and loading). FIG. 2illustrates one non-limiting example of the state-of-the-art method ofBOG recovery.

Feed stream 101 is introduced into hydrogen generation unit 102.Hydrogen generation unit 102 may be any system known in the art, such asa steam methane reformer (SMR) or Electrolysis. Hydrogen generation unit102 generates hydrogen output stream 103. Hydrogen output stream 103 mayalso come from an industrial complex off gas or from a pipeline. Oncefree of any components that may freeze in the downstream cold box (notshown), hydrogen output stream 103 passes through hydrogen output streamflow control valve 104, the function of which will be described below.Controlled hydrogen output stream 105 then enters the liquefier.

In FIG. 2 , the liquefier is based on a cycle similar to the onedescribed above, with a pre-cooling cycle (typically nitrogen) usinghigher-temperature cold box 106 and a nitrogen refrigeration cycle withnitrogen compression 107 and a hydrogen liquefaction cycle usinglower-temperature cold box 109 and a hydrogen refrigeration cycle whichincludes low-pressure hydrogen compressor 110 and high-pressure hydrogencompressor 111. Once liquefied, liquid hydrogen stream 112 goes toliquid hydrogen storage 113, which is typically a vacuum insulatedstorage tank(s) or a sphere depending on the capacity. Downstream liquidhydrogen stream 112 is the loading area which includes multiple loadingbays 115 and sometimes transfer pumps. (not shown) Liquid hydrogenstream 114 thus exits liquid hydrogen storage 113 and enters loading bay115.

FIG. 2 also shows the typical main BOG routings. For example, the BOG120 generated between lower-temperature cold box 109 and through liquidhydrogen storage 113 is generally recycled back into lower-temperaturecold box 109. Downstream liquid hydrogen stream 112, at loading bay 115,the trucks liquid and vapor phases are both connected to the plant (notshown). Hence, the vapor phase from the truck (i.e. BOG) is collectedthrough the BOG network, and sent via cool boil-off gas stream 116. Coolboil-off gas stream 116 is typically warmed up through BOG heater 117and recycled via warm boil-off gas stream 118 to low-pressure hydrogencompressor 110. The BOG flow is then mixed with the hydrogen cycle flow.

First flow indicator 119 senses the flowrate of boil-off gas flowingthrough warm boil-off gas stream 118 and re-entering low-pressurehydrogen compressor 110. First flow indicator 119 then sends a signal tohydrogen output stream flow control valve 104 which may then adjust toregulate the total hydrogen flowrate through the liquefier. Note, it isshown that first flow indictor 119 sends a signal directly to hydrogenoutput stream flow control valve 104, but the skilled artisan willrecognize that this communication may be controlled by a local computer,distributed control system (DCS), a programmable logic controller (PLC),or other systems known in the art.

FIG. 3 illustrates another non-limiting example of the state-of-the-artmethod of BOG recovery. In this example, the hydrogen is produced at arelatively low pressure (for example by using an electrolyzer) and thenintroduced into a feed compressor upstream of the liquefier. The maindifference between this example and the one presented in FIG. 2 is thatthe BOG from the loading bays and truck loading is recycled into thefeed line, instead of the hydrogen refrigeration loop.

This cycle has the advantage of recycling the BOG without impacting theliquefier operation. Another advantage is that the BOG flow does notaffect the rated flow of the feed compressor train, since the hydrogengeneration unit can be turned down to ensure that the feed compressorexperiences a constant flowrate. The feed compressor may be located evenfurther from the LP compressor and the distance between the loading areaand the feed compressor may be a design concern since the maximumallowable pressure drop may be high, as the return BOG pressure may bevery low (e.g. 1-2 bara).

Feed stream 101 is introduced into hydrogen generation unit 102.Hydrogen generation unit 102 may be any system known in the art, such asa steam methane reformer (SMR) or Electrolysis. Hydrogen generation unit102 generates hydrogen output stream 103. Hydrogen output stream 103 mayalso come from an industrial complex off gas or from a pipeline. Oncefree of any components that may freeze in the downstream cold box (notshown), Hydrogen output stream 103 passes through hydrogen output streamflow control valve 104, the function of which will be described below.Controlled hydrogen output stream 105 then combines warm boil-off gasstream 118 thus forming compressor feed stream 301. Compressor feedstream 301 is then compressed in feed compressor 302, thus formingcompressed feed stream 303, which enters the liquefier.

Again, in FIG. 3 , the liquefier is based on a cycle similar to the onedescribed above, with a pre-cooling cycle (typically nitrogen) usinghigher-temperature cold box 106 and a nitrogen refrigeration cycle withnitrogen compression 107 and a hydrogen liquefaction cycle usinglower-temperature cold box 109 and a hydrogen refrigeration cycle whichincludes low-pressure hydrogen compressor 110 and high-pressure hydrogencompressor 111. Once liquefied, liquid hydrogen stream 112 goes toliquid hydrogen storage 113, which is typically a vacuum insulatedstorage tank(s) or a sphere depending on the capacity. Downstream liquidhydrogen stream 112 is the loading area which includes multiple loadingbays 115 and sometimes transfer pumps. (not shown) Liquid hydrogenstream 114 thus exits liquid hydrogen storage 113 and enters loading bay115.

FIG. 3 shows the typical main BOG routings. For example, the BOG 120generated between lower-temperature cold box 109 and through liquidhydrogen storage 113 is generally recycled back into lower-temperaturecold box 109. Downstream liquid hydrogen stream 112, at loading bay 115,the trucks liquid and vapor phases are both connected to the plant (notshown). Hence, the vapor phase from the truck (i.e. BOG) is collectedthrough the BOG network, and sent via cool boil-off gas stream 116. Coolboil-off gas stream 116 is typically warmed up through BOG heater 117and recycled via warm boil-off gas stream 118 to compressor feed stream301. The BOG flow is then mixed with the hydrogen cycle flow.

First flow indicator 119 senses the flowrate of boil-off gas flowingthrough warm boil-off gas stream 118 and then sends a signal to hydrogenoutput stream flow control valve 104 which may then adjust to regulatethe total hydrogen flowrate through the liquefier. Note, it is shownthat first flow indictor 119 sends a signal directly to hydrogen outputstream flow control valve 104, but the skilled artisan will recognizethat this communication may be controlled by a local computer,distributed control system (DCS), a programmable logic controller (PLC),or other systems known in the art.

SUMMARY

A method for recovering boil-off gas from a system including one or moreliquefaction trains, the one or more liquefaction trains includingtransport trucks or loading bays, a gaseous hydrogen feed stream, alower-temperature cold box, and a low-pressure hydrogen compressor. Themethod including collecting a boil-off gas stream from the transporttrucks or loading bays, determining the pressure of the boil-off gasstream, and depending on the pressure, recycling the boil-off gas streamto predetermined destinations. Wherein the boil-off gas stream haseither a low-pressure, having a pressure of less than 2 bara, or amedium-pressure, having a pressure equal to or greater than 2 bara.

A method for recovering boil-off gas from a system including one or moreliquefaction trains, the one or more liquefaction trains comprisingtransport trucks or loading bays, a gaseous hydrogen feed stream, and alow-pressure hydrogen compressor. The method including collecting aboil-off gas stream from the transport trucks or loading bays, andrecycling at least a portion of the boil-off gas stream to either thelow-pressure hydrogen compressor or the gaseous hydrogen feed stream, orboth.

BRIEF DESCRIPTION OF THE FIGURES

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 is a schematic representation of a typical hydrogen liquefactionprocess, as known in the art.

FIG. 2 is a schematic representation of typical BOG routings in ahydrogen production facility, hydrogen liquefaction facility, and liquidhydrogen loading station, as known in the art.

FIG. 3 is another schematic representation of typical BOG routings in ahydrogen production facility, hydrogen liquefaction facility, and liquidhydrogen loading station, as known in the art.

FIG. 4 is a schematic representation of a hydrogen production facility,hydrogen liquefaction facility, and liquid hydrogen loading station,accordance with one embodiment of the present invention.

FIG. 5 is another schematic representation of a hydrogen productionfacility, hydrogen liquefaction facility, and liquid hydrogen loadingstation, accordance with one embodiment of the present invention.

FIG. 6 is still another schematic representation of a hydrogenproduction facility, hydrogen liquefaction facility, and liquid hydrogenloading station, accordance with one embodiment of the presentinvention.

ELEMENT NUMBERS

101=feed stream

102=hydrogen generation unit

103=hydrogen stream

104=hydrogen stream flow control valve

105=gaseous hydrogen feed stream

106=higher-temperature cold box

107=nitrogen compressor

108=cold hydrogen stream

109=lower-temperature cold box

110=low-pressure hydrogen compressor

111=high-pressure hydrogen compressor

112=liquid hydrogen stream (into storage)

113=liquid hydrogen storage unit

114=liquid hydrogen stream (out of storage)

115=loading bay

116=cool boil off gas (from loading bay)

117=boil off gas heater

118=warm boil off gas

119=first flow indicator

120=boil off gas (from liquid hydrogen storage)

301=compressor feed stream

302=feed compressor

303=compressed feed stream

401=boil off gas temperature indicator

402=boil off gas pressure indicator

403=first portion (of cool boil off gas)

404=second portion (of cool boil off gas)

405=third portion (of cool boil off gas)

406=first LP flow control valve

407=MP flow control valve

408=second LP flow control valve

409=LP boil off gas heater

410=MP boil off gas heater

411=warm LP boil off gas stream

412=warm MP boil off gas stream

413=second flow indicator

414=feed bypass stream

415=feed bypass valve

501=feed stream

502=hydrogen generation unit

503=hydrogen output stream

503A-E=compressor feed streams

504A-E=feed compressors

505=compressed feed stream

505 A-E=compressed feed stream s

506=purification unit

507=purified stream

508=portion of purified stream buffer)

509=buffer compressor

510=compressed buffer stream

511=buffer tank

512=outlet buffer stream

513=buffer valve

514=regulated buffer stream

515=combined purified stream

515A-E=inlet streams

601=portion of warm BOG stream (to low-pressure hydrogen compressor)

602=second portion of warm BOG stream

603=BOG compressor

DESCRIPTION OF PREFERRED EMBODIMENTS

Illustrative embodiments of the invention are described below, While theinvention is susceptible to various modifications and alternative forms.specific embodiments thereof have been shown by way of example in thedrawings and are herein described in detail. It should be understood,however, that the description herein of specific embodiments is notintended to limit the invention to the particular forms disclosed, buton the contrary, the intention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims.

It will of course be appreciated that in the development of any suchactual embodiment, numerous implementation-specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem-related and business-related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure.

The present schemes maximize the recovery of Liquid Hydrogen BOG byvalorizing these hydrogen molecules in one of three different ways:

-   -   By sending the BOG to the liquefier(s) (depending on the global        plant configuration several liquefiers train may be implemented)        after being compresses and re-liquefied as necessary,    -   By sending the BOG upstream of the liquefier(s), specifically at        compression trains inlet, and    -   By valorizing cold BOG directly in the liquefier to benefit from        its inherent cold.

For such a configuration, one important aspect of the present system isseparating the BOG depending on their pressure level, and temperatureconditions and designing the compression trains capacities inconsistency with the truck operations. The process control philosophy ofthe overall system is based on the adaptability of the hydrogenfeedstock load to follow BOG generation and BOG recycle.

The present system allows for a reduction of the capacity of theupstream hydrogen generation system (e.g. electrolyzers or steam methanereformer) by controlling the feed flow rate (at the inlet of thehydrogen liquefaction system) with the BOG flow rate (by turning downthe production capacity of hydrogen generation unit (or Feed flow rate)simultaneously with the recovery of BOG). The combination of thesedifferent BOG recovery options on the same site also improves the mutualflexibility of the different hydrogen compressors by separating the BOGdepending on their pressure level and temperature conditions in order toroute them to different tie-in points with different inlet pressure tomaximize the flexibility of the integrated liquefaction system and speedup the filling sequence of the trucks or increase the recovery of BOG.Finally, the speeding up of the filling sequence will allow for areduction of the number of truck loading bays.

This document primarily focuses on the BOG generation and recovery fromthe loading area, which is located downstream the hydrogen liquefactionunit and downstream the liquid hydrogen storage unit. The BOG generatedin the loading area presents many challenges, for example the BOG amountgenerated when a truck returns to the loading bay depends on thelogistics chain, delivery method, distance to customers, number ofcustomers delivered, billing method etc. As used herein, the terms“loading area” or “loading” are understood to apply to truck loading.However, one skilled in the art can recognize that the present systemmay be equally applicable to the loading of ships, or any other meansfor transporting hydrogen.

Examples of potential sources of BOG generation may be as diversified asthe following operations:

-   -   Truck depressurization, In the case of a truck arriving on site        at high pressure with a residue of liquid H2. The truck        depressurization is the step that generates the highest peak        flow of BOG    -   Truck filling. This operation will generate gaseous hydrogen        return at low pressure, and    -   “natural heat release” around the storage systems.

Valorizing all the types of BOG simply according to FIG. 2 , withinliquefier by reducing the load of the hydrogen generation system havethe following disadvantages:

-   -   A significant overdesign of the liquefier's compression train(s)        is required to be able to handle the peak BOG flow, especially        during truck depressurization and which could limit the turndown        on the liquefier's machines,    -   A process upset on the liquefier due to management of this        additional peak.

Managing the BOG only with solution principle described on FIG. 3 ,would also be challenging as it would lead to:

-   -   Depending on the suction pressure conditions of the Feed        compressor, it may be impossible to recover the low-pressure BOG        generated during the filling stage. Hence, this scheme may only        allow for partial BOG recovery, forcing a wide range of ramp        up/ramp down on feed gas compression train(s). Note that the        addition of a buffer to handle this ramp/up difficulty may be        cost prohibitive. In the current state-of-the-art, BOG recovery        with such schemes is typically limited to ˜70%. It is important        to note that when the BOG is to be recycled to the liquefier,        the feed gas flowrate is to be reduced by the same amount.        Hence, BOG recovery is even more valuable when the hydrogen        generation comes at a high OPEX cost such as electrolysis. The        H2 generation process via electrolysis is generally operated at        low pressure and a feed gas compressor ir generally required        before the hydrogen liquefier. Hence the following description        is particularly suited when the Hydrogen is generated via        electrolysis.

Turning to FIG. 4 , one embodiment of the present invention ispresented. This proposed system is typically applicable for liquefactiontrains from about 5 to above 100 tpd. The invention consists of creatingtwo different BOG networks and separating the high-pressure network fromthe low-pressure BOG recovery network. The BOG generated during truckfilling (<2 bara) is typically a steady flow at low pressure and canusually simply be warmed up and routed to the warm end of the LP cyclecompressor, similarly, to FIG. 2 . Note that during the filling stage,the BOG recovery pressure in that network is low and so is the BOGtemperature. Hence, it could also be envisaged to recover this LP BOG atthe cold end of the liquefier, in a similar fashion as what is done onthe storage BOG. The BOG generated during the depressurization is at ahigher pressure (˜2 to 10 bara) and may be much warmer, It is warmed upand routed to the feed gas compressor located upstream of the liquefier.Ideally, each low pressure and high-pressure BOG network can beconnected to every loading bay.

Hydrogen feed stream 101 is introduced into hydrogen generation unit102.

Hydrogen generation unit 102 may be any system known in the art, such asa steam methane reformer (SMR) or Electrolysis. Hydrogen generation unit102 generates hydrogen output stream 103. Hydrogen output stream 103 mayalso come from an industrial complex off gas or from a pipeline. Oncefree of any components that may freeze in the downstream cold box(purification system not shown), but typically downstream of thecompressors), Hydrogen output stream 103 passes through hydrogen outputstream flow control valve 104, the function of which will be describedbelow. Controlled hydrogen output stream 105 then combines warm MP BOGstream 412 thus forming compressor feed stream 301. Compressor feedstream 301 is then compressed in feed compressor 302, thus formingcompressed feed stream 303, which enters the liquefier. While,typically, the medium-pressure BOG is at a lower pressure than requiredat the inlet of the liquefier, compressor feed stream 301 may, ifnecessary or desired, bypass feed compressor 302 by means of feed bypassstream 414 and feed bypass valve 415.

Again, in FIG. 4 , the liquefier is based on a cycle similar to the onedescribed above, with a pre-cooling cycle (typically nitrogen) usinghigher-temperature cold box 106 and a nitrogen refrigeration cycle withnitrogen compressor 107 and a hydrogen liquefaction cycle usinglower-temperature cold box 109 and a hydrogen refrigeration cycle whichincludes low-pressure hydrogen compressor 110 and high-pressure hydrogencompressor 111. Once liquefied, liquid hydrogen stream 112 goes toliquid hydrogen storage 113. which is typically a vacuum insulatedstorage tank(s) or a sphere depending on the capacity. Downstream liquidhydrogen stream 112 is the loading area which includes multiple loadingbays 115 and sometimes transfer pumps. (not shown) Liquid hydrogenstream 114 thus exits liquid hydrogen storage 113 and enters loading bay115.

FIG. 4 shows the typical main BOG routings. For example, the BOG 120generated between lower-temperature cold box 109 and through liquidhydrogen storage 113 is generally recycled back into lower-temperaturecold box 109. Downstream liquid hydrogen stream 112, at loading bay 115,the trucks liquid and vapor phases are both connected to the plant (notshown). Hence, the vapor phase from the truck (i.e. BOG) is collectedthrough the BOG network, and sent via cool BOG stream 116.

Cool BOG stream 116 is split into at least two of the three portionspresented herein. The first portion of cool BOG stream portion 403, theflowrate of which is controlled by first LP flow control valve 406, iswarmed up through LP BOG heater 409 and recycled via warm LP BOG stream411 to low-pressure hydrogen compressor 110. The second portion of coolBOG stream portion 404, the flowrate of which is controlled by MP flowcontrol valve 407, is warmed up through MP BOG heater 410 and recycledvia warm MP BOG stream 412 to compressor feed stream 301. The BOG flowis then mixed with the hydrogen cycle flow. The third portion of coolBOG stream portion 405, the flowrate of which is controlled by second LPflow control valve 408, may be recycled back into lower-temperature coldbox 109 as an alternative path to first portion 403.

First flow indicator 119 senses the flowrate of BOG flowing through warmMP BOG stream 412 and then sends a signal to hydrogen output stream flowcontrol valve 104. Second flow indicator 413 senses the flowrate of BOGflowing through warm LP BOG stream 411 and then sends a signal tohydrogen output stream flow control valve 104. Hydrogen output streamflow control valve 104 then adjusts to regulate the total hydrogenflowrate through the liquefier. Note, it is shown that first flowindictor 119 sends a signal directly to hydrogen output stream flowcontrol valve 104, but the skilled artisan will recognize that thiscommunication may be controlled by a local computer, distributed controlsystem (DCS), a programmable logic controller (PLC), or other systemsknown in the art.

Turning to FIG. 5 , another embodiment of the present invention ispresented. FIG. 5 is similar to FIG. 4 except that it includes 2liquefaction trains, One skilled in the art will recognize that whiletwo trains are illustrated for simplicity, this process scheme will beequally applicable to systems with more than 2 trains. The BOG systemfor both trains are connected. The benefit of the invention when thereare multiple liquefaction trains is that the BOG recovery isquasi-independent of whether or not one liquefaction train is shut down.

FIG. 5 illustrates another example of the present system of BOGrecovery. In this example, the hydrogen may be produced at a relativelylow pressure (for example by using an electrolyzer) and then introducedinto a feed compressor upstream of the liquefier. This system may alsobe used if the hydrogen is produced at higher pressure by bypassing thefeed compressor.

This process scheme is typically applicable for liquefaction trains fromabout 5 to about 100 tpd and consists of producing two different BOGnetworks and separating the high pressure from the low-pressure truckBOG recovery networks. The BOG generated during the filling of the truck(<2 bara) is routed either to the warm end of the LP cycle compressor,similar to FIG. 2 . Since the BOG recovery pressure in that network islimited, the BOG temperature leaving the truck is better managed. Hence,it is much easier to recover the LP BOG at the cold end of theliquefier, in a similar fashion as what is done on the storage BOG. TheBOG generated during the depressurization are at a higher pressure (˜2to 10 bara) and is routed to the feed gas compressor. Each BOG networkmay be connected to every loading bay.

Feed stream 501 is introduced into hydrogen generation unit 502.Hydrogen generation unit 502 may be any system known in the art, such asa steam methane reformer (SMR) or Electrolysis. Hydrogen generation unit502 generates hydrogen output stream 503. Hydrogen output stream 503 mayalso come from an industrial complex off gas or from a pipeline. Oncefree of any components that may freeze in the downstream cold box (notshown), Hydrogen output stream 503 combines with warm MP BOG stream 412and is split into multiple compressor feed streams 503A-503E. It shouldbe noted that while 5 separate compressor streams are indicated, oneskilled in the art will recognize that this number may be as few as 2 oras many as necessary for the design of the system. Compressor feedstreams 503A-503E are then compressed in feed compressors 504A-504E,thus forming compressed feed streams 505A-505E, which combine intocompressed feed stream 505 which may then enter purification unit 506.

Purification unit 506 produce purified stream 507, which then enters oneor more liquefaction trains A/B. It should be noted that while 2separate liquefaction trains are indicated, one skilled in the art willrecognize that this number may be many as necessary for the design ofthe system. At least a portion 508 of purified stream 507 may beintroduced into buffer compressor 509, thereby producing compressedbuffer stream 510, which may then enter buffer tank 511. Buffer tank511, as needed, may the release outlet buffer stream 512, the flowrateof which may be regulated by buffer valve 513, thus producing regulatedbuffer stream 514, which may combine with purified stream 507 as needed,thus forming combined purified stream 515, which is split into inletstreams 515A/B, which the then enter multiple liquefaction trains A/B

Again, in FIG. 5 , each liquefier is based on a cycle similar to the onedescribed above, with a pre-cooling cycle (typically nitrogen) usinghigher-temperature cold boxes 106A/B and a nitrogen refrigeration cycleswith nitrogen compressors 107A/B and a hydrogen liquefaction cycle usinglower-temperature cold boxes 109A/B and a hydrogen refrigeration cyclewhich includes low-pressure hydrogen compressors 110A/B andhigh-pressure hydrogen compressors 111A/B. Once liquefied, liquidhydrogen streams 112A/B are combined into liquid hydrogen stream 112which goes to liquid hydrogen storage 113. Liquid hydrogen storage 113is typically a vacuum insulated storage tank(s) or a sphere depending onthe capacity. Downstream liquid hydrogen stream 112 is the loading areawhich includes multiple loading bays 115 and sometimes transfer pumps(not shown). Liquid hydrogen stream 114 thus exits liquid hydrogenstorage 113 and enters loading bay 115.

FIG. 5 shows the typical main BOG routings. For example, the BOG 120generated between lower-temperature cold box 109 and through liquidhydrogen storage 113 is split into BOG streams 120A/B recycled back intolower-temperature cold boxes 109A/B respectively. Downstream liquidhydrogen stream 112, at loading bay 115, the trucks liquid and vaporphases are both connected to the plant (not shown), Hence, the vaporphase from the truck (i.e. BOG) is collected through the BOG network,and sent via cool BOG stream 116.

Cool BOG stream 116 is split into at least two portions. The firstportion of cool BOG stream portion 403, the flowrate of which iscontrolled by first LP flow control valve 406, is warmed up through LPBOG heater 409, thus producing warm LP BOG stream 411. Warm LP BOGstream 411 is then split into warm LP BOG streams 411A/B which arerecycled to low-pressure hydrogen compressors 110A/B. The second portionof cool BOG stream portion 404, the flowrate of which is controlled byMP flow control valve 407, is warmed up through MP BOG heater 410 andrecycled via warm MP BOG stream 412 to compressor feed stream 503.

Turning to FIG. 6 , another embodiment of the present invention ispresented. FIG. 6 is an alternative solution to FIG. 4 above. In FIG. 6, the LP and HP BOG streams are combined similarly to FIG. 2 or FIG. 3 .Since the average BOG flow of a truck during an entire loading sequenceis relatively small, and depressurization BOG peaks are more rare, itmay make sense that the BOG is preferentially sent to the LP compressor.In the case of peak BOG flow, for example during depressurization of atruck, BOG compressor 603 may be added in order to recycle the extra BOGflow at the hydrogen liquefaction system feeds inlet by increasing thepressure of the extra BOG flow up to the suction pressure of the feedsystem (typically in the above scheme, up to the suction of the feedcompressor).

FIG. 6 illustrates another example of the present system of BOGrecovery. In this example, the hydrogen may be produced at a relativelylow pressure (for example by using an electrolyzer) and then introducedinto a feed compressor upstream of the liquefier. This system may alsobe used if the hydrogen is produced at higher pressure by bypassing thefeed compressor.

This process scheme is typically applicable for liquefaction trains fromabout 5 to about 100 tpd and consists of producing one single BOGnetwork. BOG generated during filling are at low pressure andpreferentially routed to the LP compressor of the liquefier. BOGgenerated during depressurization are at a higher pressure and also muchhigher flowrate. These BOG are all collected and letdown to the samelow-pressure level and recycled preferentially to the LP compressor. Theextra capacity that cannot be handled by the LP compressor (for exampleduring a peak of BOG) is routed to the BOG compressor and/or feed gascompressor. Depending on the suction conditions of feed gas compressor302, BOG compressor 603 may or may not be required.

Hydrogen feed stream 101 is introduced into hydrogen generation unit102. Hydrogen generation unit 102 may be any system known in the art,such as a steam methane reformer (SMR) or Electrolysis. Hydrogengeneration unit 102 generates hydrogen output stream 103. Hydrogenoutput stream 103 may also come from an industrial complex off gas orfrom a pipeline. Hydrogen output stream 103 then combines warm BOGstream 602 thus forming compressor feed stream 301. Compressor feedstream 301 is then compressed in feed compressor 302, thus formingcompressed feed stream 303, which enters the liquefier.

Again, in FIG. 6 , the liquefier is based on a cycle similar to the onedescribed above, with a pre-cooling cycle (typically nitrogen) usinghigher-temperature cold box 106 and a nitrogen refrigeration cycle withnitrogen compressor 107 and a hydrogen liquefaction cycle usinglower-temperature cold box 109 and a hydrogen refrigeration cycle whichincludes low-pressure hydrogen compressor 110 and high-pressure hydrogencompressor 111. Once liquefied, liquid hydrogen stream 112 goes toliquid hydrogen storage 113, which is typically a vacuum insulatedstorage tank(s) or a sphere depending on the capacity. Downstream liquidhydrogen stream 112 is the loading area which includes multiple loadingbays 115 and sometimes transfer pumps, (not shown) Liquid hydrogenstream 114 thus exits liquid hydrogen storage 113 and enters loading bay115.

FIG. 6 shows a typical main BOG routing. For example, the BOG 120generated between lower-temperature cold box 109 and through liquidhydrogen storage 113 is generally recycled back into lower-temperaturecold box 109. Downstream liquid hydrogen stream 112, at loading bay 115,the trucks liquid and vapor phases are both connected to the plant (notshown). Hence, the vapor phase from the truck (i.e. BOG) is collectedthrough the BOG network, and sent via cool BOG stream 116.

Cool BOG stream 116 is warmed up through BOG heater 117. At least aportion 601 of warm BOG stream 118 is recycled to low-pressure hydrogencompressor 110. Another portion 602 of warm BOG stream 118 is then mixedwith the hydrogen output stream 103.

ADVANTAGES

Operating range of the LP compressor is much narrower and LP compressordesign gets simpler. The LP compressor can easily operate considering 0BOG returning to the liquefier but also can well perform if multipletrucks are being filled simultaneously. The ramp-up and down between therunning cases with or without BOG recovering is therefore reduced andthe reactivity of the LP cycle compressor is enhanced

The liquefier production capacity becomes independent from the truck BOGrecycle to the LP machine since LP BOG represent a much smaller and muchmore stable flow

As mentioned above, the Feed gas compressor system design can handle thedepressurization BOG by reducing the production of the upstream H2generation unit The BOG flow is typically much smaller compared to thenominal capacity of the feed compressor; hence the depressurization BOGflow can be increased and the depressurization time decreased. That way,the entire loading process duration might be reduced to the point thatit may be possible to consider one less loading bay for the design ofthe plant. The ramping up and down of the integrated system to recoverdepressurization BOG is therefore beneficiating from the mutualizedflexibility of the hydrogen generation system as well as from the Feedcompressor.

BOG recovery is maximized (up to more than 90% recovery) and the sizingof the equipment is very little dependent on the BOG flowrate recycled.

What is claimed is:
 1. A method for recovering boil-off gas from asystem comprising one or more liquefaction trains, the one or moreliquefaction trains comprising: transport trucks or loading bays, agaseous hydrogen feed stream, a lower-temperature cold box, and alow-pressure hydrogen compressor; the method comprising: collecting aboil-off gas stream from the transport trucks or loading bays,determining the pressure of the boil-off gas stream, and depending onthe pressure, recycling the boil-off gas stream to predetermineddestinations, wherein the boil-off gas stream has either a low-pressure,having a pressure of less than 2 bara, or a medium-pressure, having apressure equal to or greater than 2 bara.
 2. The method of claim 1,wherein at least a portion of the low-pressure boil-off gas stream isrecycled to the lower-temperature cold box.
 3. The method of claim 1,wherein at least a portion of the low-pressure boil-off gas stream isrecycled to the low-pressure hydrogen compressor.
 4. The method of claim1, wherein at least a portion of the medium-pressure boil-off gas streamis recycled to the gaseous hydrogen feed stream, thereby forming acompressor feed stream.
 5. The method of claim 4, further comprising: ahydrogen generation unit configured to produce a gaseous hydrogenstream, a hydrogen stream flow control valve, and a first boil-off gasflow indicator, wherein the first boil-off gas flow indicator detectsthe flowrate of medium-pressure boil-off gas stream, and sends a signalto the hydrogen stream flow control valve, thereby adjusting theflowrate of the gaseous hydrogen stream such that the flowrate of thecompressor feed stream remains constant.
 6. The method of claim 1,further comprising a liquid hydrogen storage unit, wherein the liquidhydrogen storage unit provides liquid hydrogen to the transport trucksor loading bays and collecting a second boil-off gas stream from theliquid hydrogen storage unit and recycling the second boil-off gasstream to the lower-temperature cold box.
 7. The method of claim 1,wherein the boil-off gas is hydrogen.
 8. A method for recoveringboil-off gas from a system comprising one or more liquefaction trains,the one or more liquefaction trains comprising: transport trucks orloading bays, a gaseous hydrogen feed stream, and a low-pressurehydrogen compressor; the method comprising: collecting a boil-off gasstream from the transport trucks or loading bays, and recycling at leasta portion of the boil-off gas stream to either the low-pressure hydrogencompressor or the gaseous hydrogen feed stream, or both.
 9. The methodof claim 8, further comprising a liquid hydrogen storage unit, and alower-temperature cold boxy, wherein the liquid hydrogen storage unitprovides liquid hydrogen to the transport trucks or loading bays andcollecting a second boil-off gas stream from the liquid hydrogen storageunit and recycling the second boil-off gas stream to thelower-temperature cold box.
 10. The method of claim 8, wherein theboil-off gas is hydrogen.